Automatic control of amplification



Nov. 9, 1937.

H. BARTELS AUTOMATIC CONTROL OF AMPLIFICATION Filed 061;. 28, 1935 U fig. 1

INVENTOR.

' 2M5 BARTELS BY 7%. WV

ATTORNEY.

Patented Nov. 9, 1937 we; of amplifiers on t m memphy, teIephonyQsound andreprod icing, radio broadcasting and the ,like, .and for .its principal object the improved apparatus and methodlof',operation whereby the -amplification .ratiof of -'such .apparatus expanded o p s .in the volume of the transmittedlsignal.,..

, Such apparatus 'uunt pne e L-jmeymume of orchestral orthe like tends toeb ceed the capacity of the apparatus orithe capacity of the record,- traci; on whichthe signal is .recorded. Ii for instanoe the natural signallevel varies betweenlglOOO the latter have to be reduced to 1:50 for makingrecordqplates. -This value. e i d y. heje i heslm distance n he noise Je ek 'Wei a es the W 535 53 14"? .1 P through t matrix tween h PQ n=--;W-

thermore, the steepness of theeut-soundjgroove should not becometoo great at. largeyolume. since in this'casetheplate no longer be played without objectionable Zidistortio'n. The conditions are still less favorable in case of the talking film where it isneo'essary inyiew of the noise of the film to reduce thera'nge of amplification to an, essentiallygreaterfextent thanfin the case of record plates.

It has been determined. that forjcontrolling the amplification range automatically without disturbance or distortion, such transmission arrangements for transmitting orchestra performances, for instance, require extremely short building up periods for amplification control; The

dying out periods may be 10to times as long as the building-up periods. For dimensioning the required time constant for the control the following points are of importance:

Upon a sudden transition from the softest to the loudest parts, there always occurs ence'ssive control to a greater or less degree, especially in sound recording arrangements. These and other disturbances are avoided if the building-up period is less than twenty thousan'dths of a second (20 m sec). This fact has been ascertained in such manner that record plates especially selected as to the most favorable performance have been played through a loudspeaker and across an amplifier. By means of a mechanically controlled switching arrangement a detector has "been placed in parallel to the loudspeaker, and in this way very high and non-linear distortions were produced. The building-up period of this detector was varied between 5 and 500 111 sec It gl fi shorter'sincejthe' observers became more atten- .rangement;ir isuch ajmanner that the amplitude of the fundamental oscillation was thereby not- .chang'ed. out with this fjarrangement p'roy'edthe conditions to be essen- Lapprdiimately'f2f0fm' the disturbing action of the artificially non-linear distortion was noticeable. non-linear distortions are often eiicounteredin practice (at ordinary excess withhigh amplitude), it is' of particular imporh, "zn liasmsau." em $15..-

: Signor to 'Ielefl'rnken Gesellschaft fiir nrahh' his} alarge humber of persons used f for;.the p to a switchingin period of approximately"50 1:660 in sec., no distortions whatsoever, were discemihle. However. it was z these tests over a longer e. thesejperiodsb'ecame essentially "tIvetO'thedIstui'banceS."

.AdditionaLtests werefcarried outwith pure toneslwhe'nebyi a-bumer tone of '800 cycles '10 per 'secondKBOWHz)" was supplied to the input ofthe. amplifier, and during a short period of time a second buzaersound of 1000 Hz was added by of thej,afore-mentionedswitching arvtially, wor'segj lili'eadyat a building-up period of control of. an amplifier, pick-up, sound-recorder and the like, the second harmonics often appears tance to so dimension the control circuit that the building up period will be less than 20 m sec. H Such short building-up periods as have not been used hithertofor automatic volume control arrangements, are not only necessary for record plate recording arrangements for physiological Yreasons, but also for purely technical reasons.

Thus if the building-up period'is assumed to be 100 m sec., and'a very loud tone of for instance 50 Hz should be recorded suddenly, this signifies that 100 m sec. will have to pass before the con trol action begins. However, during 100 :11 sec. already 10 alternations of the tone of v50 Hz have passed so thatat maximum the band oi the rec- .ord plate between two grooves has been broken .through IOtimes. Hence the record is useless. At a building-up period of 20 m sec. only the first half wave has practically been recorded into the adjacent groove whereas the second alternation has been already controlled and in all probability the band between the grooves of the record plate will no longer be destroyed. It will be apparent from the foregoing that in order to eifectively prevent cutting it through the band 50 between the grooves at more than one point, the volume control should respond within a time of p the same order'of magnitude as the time of t of the time of a sound wave in order to prevent the other half wave from over-cutting.

rectifiers are utilized as control devices.

In conflict with the requirement of providing short time constants in the control arrangements, is the demand of noiseless performance. To smoothen the performance it is necessary to use filter elements of comparatively large dimensions. However, such filter elements introduce a large time constant into the control arrangement. Hence filter elements of smaller dimensions should be used whereby, however, only insufllcient filtering can be obtained. The appearing disturbing voltages are then to be eliminated in a different manner. In accordance with the present invention this is accomplished in that the control elements are applied to the amplifier in the form of a bridging circuit. With this bridging circuit the upper harmonics appearing due tolnsufilcient filtering as well as other disturbing voltages are prevented from passing into the useful circuit. To avoid unsymmetrical conditions among the elements forming the bridging circuit the one or the other one of the elements is rendered adjustable.

The invention will be better understood from the following description when considered in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring to the drawing:

Fig. 1 is a wiring diagram of an apparatus which operates on the straight part of its characteristic operating curve at high current to reduce the amplification ratio, and

Fig. 2 is a wiring diagram which operates similarly to reduce the amplification ratio.

Fig. 1 illustrates one form of the invention in which non-linear resistances in the form of dry It is obvious that the said rectifiers may also be replaced by other non-linear resistances or by tubes, in which case the circuit is to be adapted to the changed operating conditions. The impulses transmitted from the generator I through a resistance 2 are branched of! and supplied to an auxiliary rectifier 3 which, in connection with the capacity 4 and resistor 5, provides an adjusting direct potential dependent upon the mean amplification level or signal volume for the control rectifiers 6 and I connected across the amplification channel. The resistor 8 forming a bridge circuit together with the control rectifiers 6 and l is divided into approximately equal parts by the tap at 9 for the purpose of forming the bridge, so that the points l0 and I I have the same potential relative to the potential at the points l2 and I3. A biasing source 14 is provided so as to enable adjustment to the proper working point of the current-voltage characteristic of the control rectifiers which is important for the dynamic control. The tap 9 of the resistor 8 is preferably made adjustable to compensate unsymmetrical conditions of the individual elements of the bridge such as unequalities of the rectifier resistances. Obviously, the resistor 8 may be substituted by two separate variable resistors.

In reducing the amplification ratio, the arrangement of Fig. 1 has the advantage that at high amperages it operates on the straight part of the detector characteristic. If high potentials are produced at the points l2 and i3 due to large volume, it happens, as canbe readily seen from the known detector characteristic, that the resistance of the detectors 6 and I i. e. the resistance of the bridge between the points l0 and II will be decreased.

In order to have the same advantages with an increase of the amplification ratio, the circuit 0! Fig. 2 may be utilized. As in the embodiment of the example according to Fig. 1, the control impulses are branched off from the input circuit and are applied to the arrangement 3, 6, 5 and I4 which furnishes the displacement or control potential. The detected impulses are applied at points 20 and 2| of the bridge circuit 22, the latter consisting of detectors 23, 24 and resistor 25 with tap 2B. In order that at large volume the arrangement operates on the straight part of the detector characteristic, the amplification train is bridged by a resistor 21.

At high amplitudes of the alternating potential the resistance of the detector bridge becomes low. Consequently the voltage drop through the re- .sistor 21 will be increased. As in the circuit according to Fig. 1, also at increase in dynamic at higher amplitudes of the alternating potential, the resistance of the detector bridge will be decreased. In other words, the straight part of the characteristic of the detectors is utilizedin both cases.

In the two examples the alternating potential required for the production of the displacement potential is derived ahead of the control member. Obviously this potential can also be tapped in back of the control device. The dimensioning oi the individual members of the arrangement of the auxiliary detector should then be altered correspondingly. In case where the branched-off control impulses should prove to be too low, suitable amplifying means can be placed at an appropriate place in front of or in back of, the control detector 3. In view of the control curves, the resistance of the detector is to be high as compared with the bridge resistance (8 and 25). Eventually, it may be useful to place in parallel to the detectors preferably variable resistorsthus providing means of influencing the characteristic of the detectors.

What is claimed is:

1. An amplification ratio control device including a bridge element comprising a resistor and a pair of rectifiers, a pair of terminals connected to said bridge element respectively at a point between said rectifiers and to an intermediate terminal of said resistor, and means operable to apply to the end terminals of said resistor a potential which varies as the volume of the signal supplied to said terminals.

2. An amplification ratio control device including a bridge element comprising a resistor and a pair of rectifiers, a pair of terminals connected to said bridge element respectively at a point between said rectifiers and to an intermediate terminal of said resistor, and means including a rectifier connected to a resistor through a potential source for applying to the end terminals of said resistor a potential which varies as the volume of the signal supplied to said terminals.

3. An amplification ratio control device including a bridge element comprising a resistor and a pair of rectifiers, a pair of terminals connected to said bridge element respectively at a point between said rectifiers and to an intermediate terminal of said resistor, and means including a rectifier connected to a resistor through a potential source for applying to the end terminals of said resistor a potential which varies in a time of not more than twenty one thousandths of a second as the volume of the signal supplied to said terminals.

4. An amplification ratio control device including a bridge element comprising a resistor and a pair of rectifiers, a pair of terminals connected to said bridge element respectively at a point between said rectifiers and to an intermediate terminal of said resistor, and means operable to apply to the end terminals of said resistor a potential which varies in a time of the same order of magnitude as the time of a sound wave as the volume of the signal supplied to said terminals.

5. An amplification ratio control device including a bridge element comprising a resistor and a pair of rectifiers, a pair of terminals connected to said bridge element respectively at a point between said rectifiers and to an intermediate terminal 01 said resistor, and means including a rectifier connected to a resistor through a potential source for applying to the end terminals of said resistor a potential which varies in a time of the same order of magnitude as the time of a sound wave as the volume of the signal supplied to said terminals.

6. An amplification ratio control device including a bridge element comprising a resistor and a pair of rectifiers, a pair of terminals connected to said bridge element respectively at a point between said rectifiers and to an intermediate terminal of said resistor, and means operable to apply to the end terminals of said resistor a potential which varies as the volume of the signal supplied to said terminals and with a building up time of not more than twenty one-thousandths of a second.

7. An amplification ratio control device including a bridge element comprising a resistor and a pair of rectifiers, a pair of terminals connected to said bri e element respectively at a point between sai rectifiers and to an intermediate terminal of said resistor, and means including a rectifier connected to a resistor through a potential source for applying to the end terminals of said resistor a potential which varies as the volume of the signal supplied to said terminals and with a building up time of not more than twenty one-thousandths of a second.

8. An amplification ratio control device including a bridge element comprising a resistor and a pair of rectifiers, a pair of terminals connected to said bridge element respectively at a point between said rectifiers and to an intermediate terminal of said resistor, and means operable to apply to the end terminals of said resistor a potential which varies as the volume of the signal supplied to said terminals and with a building up time of the order of the time of a single sound vibration.

9. An amplification ratib control device including a bridge element comprising a resistor and a pair of rectifiers, a pair of terminals connected to said bridge elementrespectively at a point between said rectifiers and to an intermediate terminal of said resistor, and means including a rectifier connected to a resistor through -a potential source for applying to the end terminals of said resistor a potential which varies as the volume of the signal supplied to said terminals and with a building up time of the order of the time of a single sound vibration.

HANS BARTELS. 

